ENH: Code cleanup of the itkMGHImageIO

The itkMGHImageIO is a nice ITKv4 compliant IO mechanism that
would better serve a larger user community by becoming a
'Remote Module' that is part of ITKv4 proper.

The intent would be that if itkMGHImageIO became a part of ITKv4,
it could be removed from Slicer and built as part of ITKv4 and that
would make it available to stand-alone versions of SimpleITK, BRAINSTools,
ANTS, and other tools. This would benefit generating stand-alone
processing tools that share capabilities from Slicer.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21982 3bd1e089-480b-0410-8dfb-8563597acbee